Objective: To determine variation in post-instrumentation working length and root area in the following rotary systems: ProTaper, RaCe, Mtwo and K3. Study design: A sample of 40 resin blocks with double curvature (at 14 and 16 mm) and a 33º angle was used. Working length was verified with a digital measure using a number 10 K file. The specific sequence for each rotary system was followed. The canal was measured between each file, and the variation noted to determine which file showed the greatest variation within a same system and the different systems were compared. 1.5 X pre and post-instrumentation microscopic photographs were taken and measurements of the area were taken with an image analysis programme. Results: The area was found to increase by the following amounts: ProTaper: 21.85 mm²; Mtwo: 20.16 mm²; K3: 17.24 mm² and RaCe: 16.09 mm² The differences in variation of the working length were: ProTaper: 0.81 mm; Mtwo: 1.07 mm; K3: 0.31 mm and RaCe: 0.81 mm Conclusions: The values for the variation in working length are clinically not very significant. All the rotary systems analysed showed a tendency to straighten the canal and eliminate the apical curvature, and the ProTaper system was found to produce the greatest modification to the canal area and structure.